In the field of industrial coding and marking codes, dating, and traceability information are applied directly onto products and/or product packaging. To achieve high quality images on a consistent basis in industrial environments, it is advantageous for the inkjet ink to have a long decap time.
Decap time is defined as the period of time that a printhead can be left uncapped at idle and still produce a high quality code when printing is restarted. Some inkjet inks suffer from the ink drying out around the nozzles resulting in poor decap behaviour and therefore a loss of print quality. Poor decap behaviour is often caused by the solvent in the ink evaporating and leaving behind non-volatile materials which at least partially obstruct the nozzles and are therefore detrimental to jetting performance.
A common method of improving the decap performance of an inkjet ink is to add non-volatile humectants to the ink to reduce the likelihood of the ink drying in the nozzles. However this can have a detrimental effect on the drying time of the ink.
Thermal inkjet (TIJ) printing is a desirable coding and marking technology as it offers significantly higher print resolutions than competing technologies in the field, such as continuous inkjet. However, with the increasing trend to use flexible plastic materials for food packaging, there is a requirement for inks to dry quickly and adhere well to these semi-porous and non-porous substrates. The majority of commercially available thermal inkjet inks are water based and, as a result, suffer from relatively long drying times and poor adhesion on semi-porous or non-porous substrates. Thus there is a need for TIJ inks containing volatile organic solvents.
The art published to date reveals the use of various additives to improve the decap behaviour of inkjet inks, including TB inks, containing solvents, colorants, binders and humectants.
International (PCT) Patent Application WO2011/041364, discloses a thermal inkjet ink composition including a volatile organic solvent, a binder resin, a dye, up to 40% of a humectant to keep the drying time low, and an additive for extending the decap time. The additive is selected form plasticisers, surfactants, aliphatic hydrocarbons, drying oils and mixtures thereof, the resulting inks exhibiting a maximum decap time of 10 minutes.
International (PCT) Patent Application WO2010/042105 discloses the use of a decap controlling additive which has a vapour pressure from 1-5 mm Hg at 25° C., has a boiling point greater than the base solvent of the ink and is selected from the group consisting of 1-methoxy-2-propanol, ethyl lactate, tert-butanol, tert-butyl acetate, 1-butanol and combinations thereof. The inks exemplified in this application exhibit a maximum decap time of 5 minutes.
U.S. Pat. No. 8,182,597 discloses a solvent based inkjet ink composition comprising at least 50 weight % volatile organic solvent, a colorant, a humectant, from 0.3 weight % to 8 weight % of a binder resin and not more than 5 weight % water, where the ink composition is suitable for use in a thermal ink jet printer and has a decap time of at least 60 seconds. Again the maximum decap time illustrated in this patent is 5 minutes.
U.S. Pat. No. 8,142,559 discloses a thermal ink jet ink comprising one or more volatile organic solvents selected from C1-C4 alcohols, C3-C6 ketones and mixtures thereof, one or more humectants wherein the humectants are present in an amount not more than 30 weight %, one or more binder resins and one or more dyes, where the ink composition is suitable for use in a thermal ink jet printer and has a decap time of at least 15 seconds. In the examples given in the patent, maximum useful decap time is limited to 60 seconds.
European Patent No 0 501 406 discloses an ink for an inkjet printer containing an oil soluble dye, a synthetic resin and an organic solvent where the synthetic resin contains a terpene-phenol copolymer resin and a styrene-acrylic copolymer resin. The terpene-phenol copolymer resin and styrene-acrylic copolymer resins are included to provide excellent jetting stability, excellent abrasion resistance, adhesion to various recording materials and, in particular, excellent spreading of the ink dots and adhesion to a recording material attached with a wax, and a recording material coated with a coating composition containing a wax. The ink described is suitable for use in a continuous inkjet printer, an environment in which de-cap performance is not generally an issue because ink is circulated continuously and, at the end of printing, the print head is generally flushed to ensure ink does not remain in the nozzle. Not surprisingly, there is no mention of humectants in EP 0 501 406 and this patent is also silent as to decap performance.
It is one object of the present invention to overcome or address the problems of prior art inkjet ink compositions or to at least provide a commercially useful alternative thereto. It is an alternative and/or additional object to provide an inkjet ink composition which is cheaper to make and/or more effective than known inkjet ink compositions.
Preferably the inkjet ink composition as described herein has improved decap properties (i.e. increased decap times) compared with known inkjet ink compositions. Alternatively, the inkjet ink composition as described herein may have equally as commercially useful decap times as known commercial inkjet ink compositions.
Preferably the thermal ink jet ink composition for a thermal inkjet printer will not only exhibit drying times in the order of less than 5 seconds on non-porous substrates, such as polypropylene, but is also capable of exhibiting decap times of at least 1 hour.